Prior to applicants' invention described in U.S. application Ser. No. 92,485, numerous attempts were made to convert various forms of carbon, including graphite, into its diamond form or other ultra-hard carbonaceous forms at ambient conditions. None of these attempts has been adequately substantiated. A valid diamond synthesis was reported in 1955, but details were not revealed until 1959 (Nature 184:1094-8, 1959). At temperatures of 1200.degree. to 2400.degree. C. and pressures ranging from 55,000 to 100,000 atmospheres or more, carbon is converted into its diamond form in the presence of transition metals (chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum) or tantalum. Higher pressures are required at higher temperatures.
Rather esoteric means were also investigated in the quest for a more convenient graphite to diamond conversion. As reported in Phys. Rev. Letters 7:367 (1961), it was taught that diamond might be obtained in less than a microsecond by the action of extremely high pressure explosive shock waves on graphite. In fact, diamonds were recovered from carbon subjected to an explosive shock and this process is now used commercially.
Epitaxial methods have also been reported where the decomposition of gases, such as methane, ethane and propane in contact with diamond powder was found to promote diamond growth. However, in performing epitaxial techniques, temperatures in the vicinity of 1300.degree. K. and pressures on the order of 10.sup.-3 to 10.sup.-4 atmospheres were found to be required.
It is obvious that the prior techniques employed in the fabrication of synthetic diamonds and other ultra-hard carbonaceous materials are at best complicated and expensive to carry out. The maintenance of extremes in temperature and pressure requires enormous energy and sophisticated equipment, which detracts from the widespread commercialization of synthetic diamond fabrication.
The invention embraced in U.S. patent application Ser. No. 92,485 represented a marked improvement over prior art techniques in calling for the production of ultra-hard particles having covalently bonded lattice structures composed largely of carbon in a method whereby aluminum carbide (Al.sub.4 C.sub.3) was reacted with a halocarbon in a hot melt system at near ambient conditions. This invention eliminated the need for operating at extremes in temperature and pressure, thus greatly reducing the complexity of the processing system.
Although the invention described and claimed in U.S. patent application Ser. No. 92,485 remains an important contribution and dramatic step forward in the production of ultra-hard particles, there were some drawbacks. For example, the invention is restricted to the use of aluminum carbide as an initial reactant. Care had to be exercised to insure that the starting aluminum carbide be free from impurities, such as free carbon or excessive free aluminum. If excess free carbon was present in the metal carbide, graphite nucleation was promoted, thus greatly diminishing the yield of ultra-hard carbonaceous particles. For this reason, it was found that aluminum carbide starting materials should ideally possess metal to carbon ratios very close to those indicated by the stoichiometric formulas, although stoichiometries with the metal to carbon ratio or the carbon to metal ratios slightly greater than theoretical values were found acceptable.
It is thus an object of the present invention to teach the fabrication of ultra-hard particles having covalently bonded lattice structures composed largely of carbon, while eliminating the drawbacks of the prior art.
It is yet another object of the present invention to produce such ultra-hard particles by employing an expanded group of reactants beyond the aluminum carbide starting material, as described in applicants' parent application.
It is yet another object of the present invention to teach the formation of ultra-hard particles having covalently bonded lattice structures which employ acetylide carbides, interstitial carbides and metal carbides as possible initial reactants in place of aluminum carbide of the parent application.